System and Method for Current Measurement in the Presence of High Common Mode Voltages

ABSTRACT

A system and method for making accurate current measurements by determining the differential voltage drop across a resistor in series with the load in the presence of large common mode voltages. A compensating voltage equal in magnitude but 180 degrees out of phase with a common mode voltage is generated and applied to a network of resistors connected to a measurement amplifier, thereby significantly reducing the magnitude of the common mode voltage at the measurement amplifier&#39;s inputs. An error correction voltage is generated and applied to the output of the measurement amplifier to compensate for errors in the values of the resistor network.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. Ser. No. 13/443,495, filed Apr.10, 2012, now U.S. Pat. No. 8,988,063, which is incorporated herein byreference.

BACKGROUND

In many electronic systems, it is desirable to measure electric currentsflowing through a load. One such application is consumer audio devices,where it is desirable to measure the load current flowing to a speakerfrom a power amplifier. The measured current information is useful fordetermining the load on the amplifier for the purposes of determiningoverload conditions, or abnormal conditions such as short circuit loadsthat can damage the power amplifier.

Typically, the current measurement is made by placing a resistor inseries with the load, and measuring the voltage drop across thisresistor. The voltage drop can be converted to a measured current usingOhm's law. The voltage drop across the measurement resistor is minimizedto reduce the impact of the measurement on the final voltage deliveredto the load. A differential operational amplifier is often used tomeasure the very small voltage drop. The differential amplifier musthave very good “common mode rejection” specifications, however, toaccurately make this measurement.

Common mode rejection is the ability of an amplifier to subtract theeffect of the same voltage applied to both its non-inverting andinverting inputs. An ideal amplifier would have an infinite common moderejection ratio, resulting in no effect at its output if the samevoltage were applied to its two inputs. In actual practice, a number offactors limit the use of such amplifiers. Firstly, these amplifiers canbe expensive. Secondly, common mode rejection ratings of operationalamplifiers are frequency dependent, often decreasing as frequencyincreases. This makes them less desirable for AC and complex audiosignal measurements. Thirdly, the range of signal levels is limited tothe power supply voltages of the measuring amplifier, makingmeasurements of higher voltage signals found in power audio applicationsdifficult or impractical.

These and other limitations of the prior art will become apparent tothose of skill in the art upon a reading of the following descriptionsand a study of the several figures of the drawing.

SUMMARY

In an embodiment, set forth by example and not limitation, a method formeasuring an electrical current flowing in a load includes providing ameasurement resistor having a first connecting terminal and a secondconnecting terminal, the load being electrically coupled to the secondconnecting terminal of the measurement resistor, the electrical currentflowing through the measurement resistor; providing a first networkresistor having a first connecting terminal and a second connectingterminal, the first connecting terminal being electrically coupled tothe first connecting terminal of the measurement resistor; providing asecond network resistor having a first connecting terminal and a secondconnecting terminal, the first connecting terminal being electricallycoupled to the second connecting terminal of the measurement resistor;providing a third network resistor having a first connecting terminaland a second connecting terminal, the first connecting terminal beingelectrically coupled to the second connecting terminal of the firstnetwork resistor; providing a fourth network resistor having a firstconnecting terminal and a second connecting terminal, the firstconnecting terminal being electrically coupled to the second connectingterminal of the second network resistor, the second connecting terminalbeing electrically coupled to the second connecting terminal of thethird network resistor; providing a first amplifier having an invertinginput terminal, a non-inverting input terminal, and an output terminal,the non-inverting input terminal being electrically coupled to thesecond connecting terminal of the first network resistor, the invertinginput terminal being electrically coupled to the second connectingterminal of the second network resistor; measuring a common mode voltageat the second connecting terminal of the measurement resistor; creatinga compensating voltage equal in magnitude to the common mode voltage 180degrees out of phase with the common mode voltage; applying thecompensating voltage to the second connecting terminal of the third andfourth network resistors; and measuring an output voltage from the firstamplifier.

In another embodiment, set forth by example and not limitation, a methodfor measuring an electrical current flowing in a load includes providinga measurement resistor having a first connecting terminal and a secondconnecting terminal, the load being electrically coupled to the secondconnecting terminal of the measurement resistor, the electrical currentflowing through the measurement resistor; providing a first networkresistor having a first connecting terminal and a second connectingterminal, the first connecting terminal being electrically coupled tothe first connecting terminal of the measurement resistor; providing asecond network resistor having a first connecting terminal and a secondconnecting terminal, the first connecting terminal being electricallycoupled to the second connecting terminal of the measurement resistor;providing a third network resistor having a first connecting terminaland a second connecting terminal, the first connecting terminal beingelectrically coupled to the second connecting terminal of the firstnetwork resistor; providing a fourth network resistor having a firstconnecting terminal and a second connecting terminal, the firstconnecting terminal being electrically coupled to the second connectingterminal of the second network resistor, the second connecting terminalbeing electrically coupled to the second connecting terminal of thethird network resistor; providing a first amplifier having an invertinginput terminal, a non-inverting input terminal, and an output terminal,the non-inverting input terminal being electrically coupled to thesecond connecting terminal of the first network resistor, the invertinginput terminal being electrically coupled to the second connectingterminal of the second network resistor; providing a DC referencevoltage; providing a second amplifier having an inverting inputterminal, a non-inverting input terminal, and an output terminal, thenon-inverting input being electrically coupled to the DC referencevoltage, the inverting input being electrically coupled to the secondconnecting terminal of the second network resistor, the output beingelectrically coupled to the second connecting terminal of the fourthnetwork resistor; and measuring an output voltage from the outputterminal of the first amplifier.

In another embodiment, set forth by example and not limitation, anapparatus for measuring an electrical current flowing in a load includesa measurement resistor having a first connecting terminal and a secondconnecting terminal, the load being electrically coupled to the secondconnecting terminal of the measurement resistor, the electrical currentflowing through the measurement resistor; a bridged power amplifierhaving a non-inverting output terminal and an inverting output terminal,the non-inverting output terminal of being electrically coupled to thefirst connecting terminal of the measurement resistor, the invertingoutput terminal being connected to the load; a first network resistorhaving a first connecting terminal and a second connecting terminal, thefirst connecting terminal being electrically coupled to the firstconnecting terminal of the measurement resistor; a second networkresistor having a first connecting terminal and a second connectingterminal, the first connecting terminal being electrically coupled tothe second connecting terminal of the measurement resistor; a thirdnetwork resistor having a first connecting terminal and a secondconnecting terminal, the first connecting terminal being electricallycoupled to the second connecting terminal of the first network resistor;a fourth network resistor having a first connecting terminal and asecond connecting terminal, the first connecting terminal beingelectrically coupled to the second connecting terminal of the secondnetwork resistor, the second connecting terminal being electricallycoupled to the second connecting terminal of the third network resistor;a first operational amplifier having an inverting input terminal, anon-inverting input terminal, and an output terminal, the non-invertinginput terminal being electrically coupled to the second connectingterminal of the first network resistor, the inverting input terminalbeing electrically coupled to the second connecting terminal of thesecond network resistor, wherein the electrical current is measured bymonitoring a voltage at the output terminal of the first operationalamplifier.

These and other embodiments, features and advantages will becomeapparent to those of skill in the art upon a reading of the followingdescriptions and a study of the several figures of the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

Several example embodiments will now be described with reference to thedrawings, wherein like components are provided with like referencenumerals. The example embodiments are intended to illustrate, but not tolimit, the invention. The drawings include the following figures:

FIG. 1 is a schematic of an example current measurement system;

FIG. 2 is a schematic of a second example current measurement system;

FIG. 3 is a schematic of a third example current measurement system;

FIG. 4 is a graph of voltages at nodes 132 and 134 in the examplecurrent measurement system; and

FIG. 5 is a graph of voltages at nodes 136 and 138 in the examplecurrent measurement system.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1 is a schematic of an example current measurement system 100. Inthis example, a power amplifier 102 drives load 120 with an AC signal.The signal is illustrated as a sine wave but, in practice, the signalcan also include complex waveforms such as music signals. The poweramplifier may be a class D switching amplifier or a linear poweramplifier, as is known to those skilled in the art.

In the example measurement system 100, power amplifier 102 is singleended, meaning the load is connected from its output (at circuit node132) and ground. A load current i flows from power amplifier 102 to load120 through measurement resistor 104. Typically, for example,measurement resistor 104 will have a value on the order of 0.1 ohm, load120 having values ranging from 2 to 16 ohms, and load current i rangingfrom milliamps to a few amperes. Load current i is determined bymeasuring the voltage drop across measuring resistor 104.

Due to the large difference in values between the load 120 and themeasurement resistor 104, a small differential voltage drop existsacross measurement resistor 104, which may be measured by subtractingtwo essentially equal and much larger voltages. A common mode voltage ispresent at circuit node 134 and the common mode voltage plus thedifferential measurement voltage is present at circuit node 132.Subtracting these two voltages yields the differential measurementvoltage across resistor 104.

Referring to FIG. 4, graph 400 illustrates the relationship between thecommon mode voltages of this example. Curve 404 represents an examplecommon mode voltage at circuit node 134. Curve 402 represents an examplevoltage at circuit node 132. The differences between the two waveformshave been exaggerated for illustration purposes, but in reality may sosmall as to be visually indistinguishable.

Returning to FIG. 1, with the example measurement system 100, thedifferential voltage across resistor 104 is developed with the aid ofoperational amplifiers 114 and 116, along with network resistors 106,108, 110, and 112. Resistors 106, 108, 110, and 112 are designed to beof essentially equal resistance values, typically, for example 300Kohms.

Operational amplifier 116, in this example, is configured to generate awaveform equal in magnitude to the common mode voltage at circuit node134, referenced to a voltage provided by DC source 118. The common modecompensation voltage generated by amplifier 116, while equal inmagnitude to the common mode voltage, is 180 degrees out of phase. Thisvoltage is present at circuit nodes 140, 142.

Referring to FIG. 5, graph 500 illustrates the relationship between anexample common mode voltage 502 and the compensation voltage 504.Applying the compensation voltage generated by amplifier 116 to circuitnode 140 and to resistors 110 and 112, results in the cancellation ofthe common mode voltages at circuit nodes 136 and 138, the inputs ofamplifier 114. Amplifier 116 holds node 138 at a reference voltage setby DC source 118, while a voltage equal to the reference voltage plusthe differential measurement voltage across resistor 104 is present atcircuit node 136. The common mode voltage across the input ofmeasurement amplifier 114 has been reduced to the reference voltage setby source 118. This can be set to any convenient value compatible withthe operation of amplifier 114, including ground (zero potential).

Returning to FIG. 1, network resistors 106, 108, 110, and 112 cangenerate errors if they are not identical in value. In practice, noresistor can be made “identical” in value to any other. While trimmingof resistors 106-112 is possible during manufacture, tolerances maystill be too large to provide acceptable error performance. To reduceerrors contributed by resistors 106-112, variable gainamplifier/attenuator 122 is provided. Amplifier 122 combines thecompensation voltage output by amplifier 116 with common mode voltage atnodes 134, 144 to produce an error correction voltage at its output 128.This voltage is summed with the output 130 of amplifier 114 at junction124 to compensate for any errors generated by the resistor network106-112.

In one example, the error correction voltage can be determined byremoving load 120 while having a suitable output voltage from poweramplifier 102. Since no current is flowing, the output from amplifier114 should be zero. Any errors in the resistors 106-112 will produce anerror voltage at the output of amplifier 114, which can be “nulled” tozero by an error correction voltage from amplifier 122, which isadjusted by altering the amplifier's gain. Since the error voltage isproportional to the magnitude of the common mode voltage, amplifier 122compensates for this effect by providing an error correction voltagethat is also proportional to the magnitude of the common mode voltage.

FIG. 2 is a schematic of another example current measurement system 200.In this example embodiment, measurement amplifier 114 in FIG. 1 isreplaced with a differential output amplifier 202 and resistors 204,206. A differential output amplifier provides two voltage outputs, eachhaving opposing polarities. Summing junction 208 is configured to sumthe (absolute) magnitudes of the two voltage outputs by phase shiftingone output by 180 degrees. This summed output is added to the errorcorrection voltage from amplifier 122 as previously described.

FIG. 3 is a schematic of a still further example current measurementsystem 300. In this example embodiment, power amplifier 302 is adifferential output amplifier producing two outputs of opposingpolarity, 180 degrees out of phase with each other. These amplifiers mayalso be referred to as “bridged” amplifiers because they can be createdfrom combining two single ended amplifiers (like amplifier 102 of FIGS.1 & 2). This is done by inverting the input on one amplifier andconnecting the load across the outputs of both amplifiers instead ofbeing grounded.

A potential advantage to the example embodiment of FIG. 3 is that thecommon mode compensating voltage is generated by the power amplifier 302directly, and is present at circuit node 308. The common mode voltage isstill present at circuit node 134, as in previously describedembodiments. Amplifier 116 and reference source 118 are not required inthis embodiment. The function and operation of amplifiers 114 and 122remain as previously described above in FIG. 1.

Although various embodiments have been described using specific termsand devices, such description is for illustrative purposes only. Thewords used are words of description rather than of limitation. It is tobe understood that changes and variations may be made by those ofordinary skill in the art without departing from the spirit or the scopeof various inventions supported by the written disclosure and thedrawings. In addition, it should be understood that aspects of variousother embodiments may be interchanged either in whole or in part. It istherefore intended that the claims be interpreted in accordance with thetrue spirit and scope of the invention without limitation or estoppel.

What is claimed is:
 1. A method for measuring an electrical currentflowing in a load comprising: providing a measurement resistor, saidresistor having a first connecting terminal and a second connectingterminal, said load being electrically coupled to said second connectingterminal of said measurement resistor, said electrical current flowingthrough said measurement resistor; providing a first network resistor,said first network resistor having a first connecting terminal and asecond connecting terminal, said first connecting terminal of said firstnetwork resistor being electrically coupled to said first connectingterminal of said measurement resistor; providing a second networkresistor, said second network resistor having a first connectingterminal and a second connecting terminal, said first connectingterminal of said second network resistor being electrically coupled tosaid second connecting terminal of said measurement resistor; providinga third network resistor, said third network resistor having a firstconnecting terminal and a second connecting terminal, said firstconnecting terminal of said third network resistor being electricallycoupled to said second connecting terminal of said first networkresistor; providing a fourth network resistor, said fourth networkresistor having a first connecting terminal and a second connectingterminal, said first connecting terminal of said fourth network resistorbeing electrically coupled to said second connecting terminal of saidsecond network resistor, said second connecting terminal of said fourthnetwork resistor being electrically coupled to said second connectingterminal of said third network resistor; providing a first amplifier,said first amplifier having an inverting input terminal, a non-invertinginput terminal, and an output terminal, said non-inverting inputterminal of said first amplifier being electrically coupled to saidsecond connecting terminal of said first network resistor, saidinverting input terminal of said first amplifier being electricallycoupled to said second connecting terminal of said second networkresistor; providing a DC reference voltage; providing a secondamplifier, said second amplifier having an inverting input terminal, anon-inverting input terminal, and an output terminal, said non-invertinginput of said second amplifier being electrically coupled to said DCreference voltage, said inverting input of said second amplifier beingelectrically coupled to said second connecting terminal of said secondnetwork resistor, said output of said second amplifier beingelectrically coupled to said second connecting terminal of said fourthnetwork resistor; measuring an output voltage from said output terminalof said first amplifier; providing a third amplifier, said thirdamplifier having a variable gain, said third amplifier having a firstinput terminal, a second input terminal, and an output terminal, saidfirst input terminal of said third amplifier being electrically coupledto said first connecting terminal of said second network resistor, saidsecond input terminal of said third amplifier being electrically coupledto said second connecting terminals of said third and said fourthnetwork resistors; generating an error correction voltage at said outputof said third amplifier by adjusting said variable gain of said thirdamplifier; and adding said error correction voltage to said outputvoltage of said first amplifier.
 2. A method as recited in claim 1comprising: flowing a reference current through said measurementresistor; adjusting said gain of said third amplifier such that a sum ofsaid error correcting voltage and said output voltage from said firstamplifier indicate said reference current is measured.
 3. A method asrecited in claim 2 wherein said reference current is approximately zero,and said gain of said third amplifier is adjusted such that said sum ofsaid error correcting voltage and said output voltage from said firstamplifier is approximately zero.
 4. An apparatus for measuring anelectrical current flowing in a load comprising: a measurement resistor,said resistor having a first connecting terminal and a second connectingterminal, said load being electrically coupled to said second connectingterminal of said measurement resistor, said electrical current flowingthrough said measurement resistor; a bridged power amplifier, said poweramplifier having a non-inverting output terminal and an inverting outputterminal, said non-inverting output terminal of said power amplifierbeing electrically coupled to said first connecting terminal of saidmeasurement resistor, said inverting output terminal of said poweramplifier being connected to said load; a first network resistor, saidfirst network resistor having a first connecting terminal and a secondconnecting terminal, said first connecting terminal of said firstnetwork resistor being electrically coupled to said first connectingterminal of said measurement resistor; a second network resistor, saidsecond network resistor having a first connecting terminal and a secondconnecting terminal, said first connecting terminal of said secondnetwork resistor being electrically coupled to said second connectingterminal of said measurement resistor; a third network resistor, saidthird network resistor having a first connecting terminal and a secondconnecting terminal, said first connecting terminal of said thirdnetwork resistor being electrically coupled to said second connectingterminal of said first network resistor; a fourth network resistor, saidfourth network resistor having a first connecting terminal and a secondconnecting terminal, said first connecting terminal of said fourthnetwork resistor being electrically coupled to said second connectingterminal of said second network resistor, said second connectingterminal of said fourth network resistor being electrically coupled tosaid second connecting terminal of said third network resistor and saidinverting output terminal of said power amplifier; and a firstoperational amplifier, said first operational amplifier having aninverting input terminal, a non-inverting input terminal, and an outputterminal, said non-inverting input terminal of said first operationalamplifier being electrically coupled to said second connecting terminalof said first network resistor, said inverting input terminal of saidfirst operational amplifier being electrically coupled to said secondconnecting terminal of said second network resistor, wherein saidelectrical current is measured by monitoring a voltage at said outputterminal of said first operational amplifier.
 5. An apparatus as recitedin claim 4 further comprising: a second operational amplifier, saidsecond operational amplifier having a variable gain, said secondoperational amplifier having a first input terminal, a second inputterminal, and an output terminal, said first input terminal of saidsecond operational amplifier electrically coupled to said non-invertingoutput of said power amplifier, said second input of said secondoperational amplifier electrically coupled to said inverting output ofsaid power amplifier; a summing junction, said summing junction having afirst input, a second input, and an output, said first input of saidsumming junction electrically coupled to said output terminal of saidfirst operational amplifier, said second input of said summing junctionelectrically coupled to said output terminal of said second operationalamplifier, wherein said electrical current is measured by monitoring avoltage at said output of said summing junction.
 6. An apparatus asrecited in claim 4 wherein said measurement resistor is approximately0.1 ohms.
 7. An apparatus as recited in claim 4 wherein said firstnetwork resistor, said second network resistor, said third networkresistor, and said fourth network resistor have values between 100K ohmsand 500K ohms.
 8. An apparatus as recited in claim 4 wherein said firstnetwork resistor, said second network resistor, said third networkresistor, and said fourth network resistor have values of approximately300K ohms.
 9. An apparatus for measuring an electrical current flowingin a load comprising: a differential output amplifier having a firstoutput and a second output of opposing polarities; a measurementresistor adapted to couple one of the first output and second output ofthe differential output amplifier to a load, whereby a currentoperationally flowing through the measurement resistor is representativeof the current flowing through the load; a resistive bridge coupledacross the measurement resistor; and a measurement amplifier coupled tothe resistive bridge, whereby the differential output amplifiergenerates a common mode compensating voltage across the load and anoutput of the operational amplifier is related to the current flowingthrough the load.
 10. An apparatus for measuring an electrical currentflowing through a load as recited in claim 9 wherein the first outputand the second output of the differential output amplifier are 180° outof phase.
 11. An apparatus for measuring an electrical current flowingthrough a load as recited in claim 10 wherein the differential outputamplifier is a bridged amplifier.
 12. An apparatus for measuring anelectrical current flowing through a load as recited in claim 11 whereinthe differential output amplifier is a power amplifier.
 13. An apparatusfor measuring an electrical current flowing through a load as recited inclaim 12 wherein the measurement amplifier is an operational amplifierhaving a non-inverting input and an inverting input.
 14. An apparatusfor measuring an electrical current flowing through a load as recited inclaim 13 wherein the resistive network includes a first voltage dividerand a second voltage divider.
 15. An apparatus for measuring anelectrical current flowing through a load as recited in claim 14 whereinthe non-inverting input of the measurement amplifier is coupled to afirst node of the first voltage divider and the inverting input of themeasurement amplifier is coupled to a second node of the second voltagedivider.
 16. An apparatus for measuring an electrical current flowingthrough a load as recited in claim 15 wherein common mode voltages areeffectively cancelled at the non-inverting input and the inverting inputof the measurement amplifier.
 17. An apparatus for measuring anelectrical current flowing through a load as recited in claim 16 furthercomprising an error correction amplifier having a first input coupled tothe first output of the differential output amplifier and a second inputcoupled to the second output of the differential output amplifier. 18.An apparatus for measuring an electrical current flowing through a loadas recited in claim 17 further comprising a summer having a first inputcoupled to the output of the measurement amplifier and a second inputcoupled to the output of the error correction amplifier.